Method and apparatus for thermo-plastic composite wood

ABSTRACT

A method of manufacturing a thermoplastic composite wood material. First, a first quantity of a first wood component of wood chips having long axes and a first size range is mixed with a second quantity of a second wood component of wood particles having a second size and a third quantity of a first thermoplastic polymer component of molten thermoplastic polymer until substantially all of the wood chips and the wood particles are encapsulated by the thermoplastic polymer. The second size range is distinct from the first size range and has substantially smaller values than the first size range. Next, the long axes of the wood chips of the first wood component are oriented such that they are substantially parallel to a predetermined plane. Next, a loose material constituting a mixture of the wood chips encapsulated in the thermoplastic polymer and the wood particles encapsulated in the thermoplastic polymer is deposited onto a press inlet feed unit while maintaining the orientation of the long axes of the wood chips. The loose material is then pressed in a direction substantially parallel to the predetermined plane such that it is compacted and such that the long axes of the wood chips are aligned substantially parallel to one another.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application is a Divisional of pending application Ser. No.09/955,364, entitled “THERMOPLASTIC COMPOSITE WOOD MATERIAL”, filed Sep.18, 2001, which application is related to, and claims the benefits ofpriority from, Provisional Application Serial No. 60/233,172, entitled“THERMO-PLASTIC COMPOSITE WOOD”, filed Sep. 18, 2000, and ProvisionalApplication Serial No. 60/257,728, entitled “THERMO-PLASTIC COMPOSITEWOOD FOR FENCES”, filed Dec. 21, 2000.

TECHNICAL FIELD OF THE INVENTION

[0002] This invention is related to a composite material comprising oneor more discontinuous phases of wood particles encapsulated in apolymeric matrix, and also methods and equipment for manufacturing same.In one aspect, it relates to a composite material comprising twodiscontinuous wood phases having distinct size ranges encapsulated in athermoplastic polymeric matrix.

BACKGROUND OF THE INVENTION

[0003] Composite materials consisting of natural wood fibers orparticles held together with a binder substance have been widely used inthe construction industry for many years and are produced on aworld-wide basis. Specific examples of these composite materials, whichmay also be referred to as “composite wood”, include: plywood, which ismade from thin sheets of virgin wood fibers pressed together with athermoset resin binder; particle board, made using finely ground virginwood particles pressed together with a thermoset resin binder; orientedstrand board, made from thin oriented virgin wood wafers pressedtogether with a thermoset resin binder; and medium and high densityfiberboard, made from virgin wood particles pressed together with athermoset resin binder.

[0004] More recently, a composite wood material has been produced usingfinely ground wood particles, also known as wood “flour”, encapsulatedin a thermoplastic matrix. The composite wood is essentially athermoplastic material mixed, or “compounded”, together with a woodflour filler material. The compounding of thermoplastics with fillermaterial has been carried out for many years to modify the physicalproperties of thermoplastics. The compounding of thermoplastics withfinely ground wood flour particles can produce a composite wood materialhaving certain physical properties which are superior to natural wood,for example, reduced water absorption, improved resistance to chemicaldegradation, improved resistance to rot, and improved resistance totermites and other wood damaging insects. Due to these superior physicalproperties, compounded thermoplastic composite wood materials arecurrently used in many applications, for example, exterior door andwindow frames, furniture, decking, boardwalks, siding and trimmaterials.

[0005] While exhibiting certain desirable physical properties,compounded thermoplastic composite wood materials currently beingproduced typically have a density that is significantly greater thannatural wood. For example, some existing thermoplastic composite woodmaterials have a density that is 60% greater than natural wood. Further,these materials are generally not recommended for use in load-bearing orstructural applications because the thermoplastic is the primarystructural component and it deforms excessively under loaded conditions.Typically, the amount of wood flour filler in the thermoplastic matrixis generally in the range of 50% by weight and the encapsulated woodparticles are about 40-60 mesh (i.e., about 0.016-0.010 inch) in size.In order to reduce the overall weight of products made using these densethermoplastic wood composite materials, the finished product designoften includes thin walls with hollow spaces and structural supportwebs. In some cases, a foaming agent is added to the thermoplastic toreduce the density of the material. However, such foaming agents andthin-wall designs can result in a significant reduction in the impactand/or shear strength properties of the products made using thesethermoplastic composite wood materials.

[0006] Notwithstanding the composite materials previously disclosed byothers, their remains a need for thermoplastic composite wood materialshaving improved density, impact resistance, flex-modulus, compressionstrength, and/or shear strength properties in comparison to currentmaterials.

[0007] A need further exists, for methods of producing thermoplasticcomposite wood materials having the improved characteristics describedabove.

[0008] A need still further exists for equipment useful in theproduction of thermoplastic composite wood materials, or alternately inprocesses where the controlled dispensing of high viscosity materials isrequired.

SUMMARY OF THE INVENTION

[0009] The present invention disclosed and claimed herein, in one aspectthereof, comprises a composite material including a first woodcomponent, a second wood component and a thermoplastic polymercomponent. The first wood component is of substantially axially alignedwood chips having a first size range and being distributed throughoutthe composite material in a first discontinuous phase. The second woodcomponent is of wood particles having a second size range and beingdistributed between the wood chips in a second discontinuous phase, thesecond size range being distinct from the first size range and havingsubstantially smaller values than the first size range. Thethermoplastic polymer component encapsulates the wood chips and the woodparticles and forms a continuous phase extending throughout thecomposite material. The wood chips constitute from about 100 to about 0weight percent of a total weight of the first and second wood componentsand the wood particles constitute from about 0 to about 100 weightpercent of the total weight of the first and second wood components. Thefirst and second wood components together constitute from about 90 toabout 50 weight percent of the composite material. The thermoplasticpolymer component constitutes from about 10 to about 50 weight percentof the composite material.

[0010] In another aspect, the invention comprises a composite materialincluding an inner structural member and an outer structural member. Theinner structural member has a first wood component of substantiallyaxially aligned wood chips having a first size range and beingdistributed throughout the inner structural member in a firstdiscontinuous phase. A second wood component of wood particles having asecond size range is distributed between the wood chips in a seconddiscontinuous phase, the second size range being distinct from the firstsize range and having substantially smaller values than the first sizerange. A first thermoplastic polymer component encapsulates the woodchips of the first wood component and the wood particles of the secondwood component and forms a first continuous phase extending throughoutthe first structural member. The first and second wood componentstogether constitute from about 90 to about 50 weight percent of theinner structural member, and the first thermoplastic polymer componentconstitutes from about 10 to about 50 weight percent of the innerstructural member. The outer structural member is continuously joined tothe inner structural member. The outer structural member includes athird wood component of wood particles having a third size range andbeing distributed throughout the outer structural member in a thirddiscontinuous phase. A second thermoplastic polymer componentencapsulates the wood particles of the third wood component and forms asecond continuous phase extending throughout the outer structuralmember. The third wood component constitutes from about 10 to about 50weight percent of the outer structural member, and the secondthermoplastic polymer component constitutes from about 90 to about 50weight percent of the outer structural member.

[0011] In yet another aspect, the invention comprises a method ofmanufacturing a thermoplastic composite wood material comprising aninner structural member including substantially axially aligned woodchips having a first size range, a second wood component of woodparticles having a second size range, and a first thermoplastic polymercomponent encapsulating the wood chips and the wood particles andforming a first continuous phase extending throughout the firststructural member. The method includes the step of mixing together afirst quantity of a first wood component of wood chips having long axesand a first size range, a second quantity of a second wood component ofwood particles having a second size range distinct from the first sizerange, and a third quantity of a first thermoplastic polymer componentof molten thermoplastic polymer until substantially all of the woodchips and the wood particles are encapsulated by the thermoplasticpolymer. It further includes the step of orienting the long axes of thewood chips of the first wood component such that they are substantiallyparallel to a predetermined plane. It further includes the step ofdepositing a loose material constituting a mixture of the wood chipsencapsulated in the thermoplastic polymer and the wood particlesencapsulated in the thermoplastic polymer onto a press inlet conveyorunit while maintaining the orientation of the long axes of the woodchips. It further includes the step of pressing the loose material in adirection substantially parallel to the predetermined plane such that itis compacted and such that the long axes of the wood chips are orientedsubstantially parallel to one another.

[0012] In still another aspect, the invention comprises a method ofmanufacturing a thermoplastic composite wood material comprising aninner structural member and an outer structural member, where the innerstructural member includes a first wood component of substantiallyaxially aligned wood chips having a first size range and beingdistributed throughout the inner structural member in a firstdiscontinuous phase, where the inner structural member also includes asecond wood component of wood particles having a second size range andbeing distributed between the wood chips in a second discontinuousphase, and where the inner structural member also includes a firstthermoplastic component encapsulating the wood chips and the woodparticles and forming a first continuous phase extending throughout theinner structural member, and further where the outer structural memberis continuously joined to the inner structural member and includes athird wood component of wood particles having a third size range and asecond thermoplastic component encapsulating the wood particles of thethird wood component and forming a second continuous phase extendingthroughout the outer structural member. The method comprises the step ofmixing together a first quantity of a first wood component of wood chipshaving long axes and a first size range, a second quantity of a secondwood component of wood particles having a second size range distinctfrom the first size range, and a third quantity of a first thermoplasticcomponent of molten thermoplastic, until substantially all of the woodchips and the wood particles are encapsulated by the thermoplastic. Itfurther includes the step of orienting the long axes of the wood chipsof the first wood component such that they are substantially parallel toa predetermined plane. It further includes the step of depositing aloose material constituting a mixture of the wood chips encapsulated inthe thermoplastic and the wood particles encapsulated in thethermoplastic onto a press inlet feed unit while maintaining theorientation of the long axes of the wood chips. It further includes thestep of pressing the loose material in a direction substantiallyparallel to the predetermined plane such that it is compacted and suchthat the long axes of the wood chips are aligned substantially parallelto one another, thereby forming an inner structural member. It furtherincludes the step of mixing together a fourth quantity of a third woodcomponent of wood particles having a third size range and a fifthquantity of a second thermoplastic component of molten thermoplasticuntil substantially all of the wood particles are encapsulated by thethermoplastic. It further includes the step of joining the materialformed by mixing the third wood component and the second thermoplasticto the inner structural member.

[0013] In still another aspect, the invention comprises a compounderunit for mixing and orienting shaped pieces within a viscous material.The compounder unit comprises an outer casing having exterior wallsdefining a longitudinal cavity therein, the cavity being subdivided intoa material inlet section, a mixing section, an orientation section, andan outlet passage and having a long axis passing therethrough. At leastone compounding shaft is positioned within the longitudinal cavityparallel to the long axis, the compounding shaft having a plurality ofblades formed thereon. The blades on a portion of compounding shaftwithin the orientation section include screw blades having a pitch whichprogressively decreases as the distance from the blade position to theoutlet passage decreases.

[0014] In still another aspect, the invention comprises a coating dieapparatus for extruding a viscous materials. The apparatus comprises anexterior casing that defines a longitudinal cavity and a shaft rotatablymounted in the cavity. The shaft has a plurality of mixing elementsmounted thereon defining a feed section and a dispensing section. Themixing elements in the feed section have a positive pitch for urgingmolten material received into the feed section of the cavity into thedispensing section of the cavity and the mixing elements in thedispensing section urge molten material received in the dispensingsection tangentially against the interior of the exterior casing. Theexterior casing further defines a dispensing slot formed tangentiallythrough the exterior casing and extending longitudinally across thedispensing section. Molten material received into the dispensing sectionis sliced off by the exposed inner edge of the casing to form a rawsheet of molten material which exits the dispensing slot.

BRIEF DESCRIPTION OF THE DRAWINGS

[0015] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings inwhich:

[0016]FIG. 1 illustrates an enlarged sectional view of a portion ofcomposite wood material in accordance with one embodiment of the currentinvention, comprising two distinct discontinuous phases of wood piecesdispersed in a continuous thermoplastic matrix, it being understood thatthe distance shown between the wood pieces is exaggerated for purposesof illustration;

[0017]FIGS. 2a and 2 b illustrate enlarged views of a representativewood chip from the first discontinuous phase, FIG. 2a being a side viewand FIG. 2b being an end view;

[0018]FIG. 3 illustrates an enlarged view of a representative woodparticle from the second discontinuous phase;

[0019]FIG. 4 is a graph showing piece size versus the percentage of allpieces in a phase having the specified size for the wood pieces in thematerial;

[0020]FIG. 5 illustrates a baffled grinding apparatus for producing woodchips suitable for the first wood component;

[0021]FIG. 6 illustrates an enlarged sectional view of a portion of acomposite wood material in accordance with another embodiment of thecurrent invention, comprising an inner structural member continuouslyjoined to an outer structural member, it being understood that thedistance shown between the wood pieces is exaggerated for purposes ofillustration;

[0022]FIG. 7 illustrates a cross-sectional view of an extrudate formedfrom the composite wood material in accordance with another embodimentin which the outer structural member completely surrounds the innerstructural member;

[0023]FIG. 8 illustrates a sectional view of another extruded member inaccordance with another embodiment in which the outer structural memberdoes not completely surround the inner structural member;

[0024]FIG. 9 is a simplified block diagram of a method for manufacturinga thermoplastic composite wood material in accordance with anotherembodiment;

[0025]FIGS. 10a and 10 b illustrate the orientation of the wood chips atthe exit of the inner compounder, FIG. 10a being a side view and FIG.10b being an end view;

[0026]FIGS. 11a and 11 b illustrate the alignment of the wood chips atthe exit of the continuous press, FIG. 11a being a side view and FIG.11b being an end view;

[0027]FIG. 12 illustrates a plan view of a plant for manufacturing thethermoplastic composite wood material;

[0028]FIGS. 13 and 14 illustrate diagrammatically the inner compounderunit, FIG. 13 being a plan view and FIG. 14 being a side view;

[0029]FIG. 15 illustrates diagrammatically the continuous press unit;

[0030]FIGS. 16a and 16 b illustrate the compression rollers of thecontinuous press unit, FIG. 16a being an end view (in the direction ofmaterial flow) and FIG. 16b being a side view;

[0031]FIG. 17 illustrates an outer profile molder for applying the outerstructural member to the inner structural member;

[0032]FIGS. 18a and 18 b illustrate an alternative applicator unit whichutilizes coating rollers to apply the outer structural member to theinner structural member;

[0033]FIGS. 19a and 19 b illustrate a coating die unit of a typesuitable for applying the outer structural member to the innerstructural member, FIG. 19a being an end view thereof and FIG. 19b beinga side view thereof; and

[0034]FIGS. 20a and 20 b illustrate cross sectional views of the coatingdie in FIGS. 19a and 19 b, FIG. 20a being an enlarged section takenalong line 20 a-20 a of FIG. 20b and FIG. 20b being a section takenalong line 20 b-20 b of a FIG. 20a.

DETAILED DESCRIPTION OF THE INVENTION

[0035] Referring now to FIG. 1, there is illustrated a thermoplasticcomposite wood material 100 in accordance with a first embodiment of thecurrent invention. The composite wood material 100 comprises a firstwood component of substantially axially aligned wood chips 102 having afirst size range and being distributed throughout the composite materialin the first discontinuous phase. A second wood component of woodparticles 104 having a second size range is distributed between the woodchips 102 and throughout the composite wood material 100 in a seconddiscontinuous phase. As is further described below, the second sizerange of the wood particles 104 is distinct from the first size range ofthe wood chips 102, and has substantially smaller values than the firstsize range. A thermoplastic polymer component encapsulates (i.e., fullycoats) the wood chips 102 and the wood particles 104, forming acontinuous phase or matrix 106 extending throughout the composite woodmaterial 100. An axis 108 may be defined passing through the material100 as shown. It will be appreciated in FIG. 1, that the distancebetween the wood chips 102 and the wood particles 104 is exaggerated forpurposes of illustration. The actual thickness of the thermoplasticcomponent 106 between the wood components must only be sufficient toencapsulate the individual wood chips 102 and wood particles 104. Thus,in the finished composite wood material the wood chips 102 and woodparticles 104 will be very tightly packed together.

[0036] Referring now to FIGS. 2a and 2 b, there is illustrated anenlarged view of a wood chip 102 of the type in the first woodcomponent. As best seen in FIG. 2a, the wood chip 102 is characterizedby a length L defined by the largest dimension of the wood chip, and bya dimension W, which is the larger of the two widths W₁ and W₂ measuredperpendicular to each other and to the length L. A long axis 202 passesthrough the wood chip 102 in a direction parallel to the length L.Typically, although not exclusively, the long axis 202 is substantiallyparallel to the grain 204 of the wood. The wood chips 102 of the firstwood component are substantially axially aligned when their long axes202 are substantially parallel to one another and to the material axis108.

[0037] Referring now to FIG. 3, there is illustrated an enlarged view ofa wood particle 104 of the type in the second wood component. The woodparticle 104 is characterized by a single dimension D, which is theparticle's largest dimension.

[0038] Referring now to FIG. 4, it is illustrated that the wood chips102 in the first wood component have a range of dimensions (e.g., L andW) within a first size range 402 between Min₁ and Max₁, and the woodparticles 104 in the second wood component have a range of dimensions(e.g., D) within a second size range 404 between Min₂ and Max₂. In otherwords, substantially all of the wood chips 102 have dimensions rangingbetween Min₁ and Max₁, and substantially all of the wood particles 104have a dimension ranging between Min₂ and Max₂. The size range of thewood particles 104 is considered “distinct” from the size range of woodchips 102 because the largest wood particles, having a size Max₂, aresubstantially smaller than the smallest wood chips, having a size Min₁.Put another way, this distinction is represented by the gap 406 betweenthe first size range 402 and the second size range 404, indicating thatthe thermoplastic composite wood material 100 comprises substantially nowood pieces having a size between Max₂ and Min₁.

[0039] The range sizes for the wood pieces in the first and second woodcomponents can vary from one embodiment to another. For example, in oneembodiment, the wood chips 102 have a first size range with lengthdimension L from about ¼ inch to about 2 inches and width dimension Wfrom about {fraction (1/16)} inch to about ¾ inch, and the woodparticles 104 have a second size range with diameter D from about 0.010inch to about 0.076 inch (i.e., from about 60 mesh to about 9 mesh). Ina more preferred embodiment, the wood chips 102 have a first size rangewith the length dimension L from about ¼ inch to about 1-½ inches and awidth dimension W from about {fraction (1/16)} inch to about ¼ inch, andthe wood particles 104 have a second size range with a diameter D fromabout 0.016 inch to about 0.046 inch (i.e., from about 40 mesh to about16 mesh). In a still more preferred embodiment, the wood chips 102 havea first size range with the length dimension L from about ¾ inch toabout 1-¼ inch and a width dimension W from about ⅛ inch to about ¼inch, and the wood particles 104 have a second size range with adiameter D from about 0.023 inch to about 0.033 inch (i.e., from about30 mesh to about 20 mesh).

[0040] Referring now to FIG. 5, there is illustrated a baffled screengrinding apparatus for producing wood chips 102 which are suitable forthe first wood component of the thermoplastic composite wood material.While it will be appreciated that wood chips 102 having the necessarydimensions may be produced by a variety of methods and apparatus, abaffled screen grinding unit as illustrated in FIG. 5 has beendetermined to be particularly effective in producing suitable chips.Scrap wood or other wood raw material 502 is fed into a cutting chamber504 of the grinding apparatus by a conveyor 506. A hold-down roller 508may be used to ensure positive feeding of the raw material. A grindingrotor 510 having a plurality of cutting teeth 512 positioned around itsperiphery is located in the cutting chamber 504 and rotated at highspeed. As the wood material 502 enters the cutting chamber 504, thecutting teeth 512 initially cut the wood against an anvil 514, thusreducing the wood to the size of the opening between the anvil and thecutting teeth. This opening is typically about ¼ the width of thecutting teeth 512 (which in this case is about 1 inch). The cuttingteeth 512 are staggered on the grinding rotor 510 to allow larger piecesto pass the anvil 514 onto a screen 516. The screen 516 is setapproximately ¼ inch away from the grinding rotor 510 so that thecutting teeth 512 grind the wood as it moves across the surface of thescreen at 516. Centrifugal force and the force of the cutting teeth 512push a plurality of precursor wood pieces 518 through the openings inthe screen 516 as they are ground. A plurality of baffles 520 are setaround the periphery of the screen 516 so that the precursor wood pieces518 passing through the screen are chopped off at a length of about 1inch long or less. The precursor wood pieces 518 are thus reduced insize by the grinding operation until they will pass through the 1 mesh(i.e., 1 inch square) openings in the screen 516. In practice, it maytake three separate grinding steps to produce wood chips 102 suitablefor the first wood component: one grind through a 2 inch square baffledscreen and two grinds through a 1 mesh (i.e., 1 inch square) baffledscreen. Each successive grind reduces the size of the wood chips 518 toa more uniform size distribution.

[0041] It will be appreciated that during the grinding operation justdescribed, wood pieces smaller than those desired for the first woodcomponent will also pass through the baffled screen along with theprecursor wood pieces 518. Accordingly, following the initial grindingto produce the precursor wood pieces 518, the resulting material is thenscreened to remove the undersized particles (also known as “fines”). Ithas been determined that a ¼0 inch or {fraction (3/16)} inch horizontalshaker screen can be used to efficiently remove the fines from theprecursor wood pieces. The exact size of shaker screen to be useddepends upon the moisture level of the wood pieces in the amount offines present. The shaker screen uses the force of gravity to move thesmaller particles through the layers of larger wood chip and ultimatelythrough the screen as the materials are shaken and/or vibrated. It hasbeen determined that as a general rule, only wood pieces with both alength dimension L and a width dimension W (i.e., both width W₁ andthickness W₂) which are less than the shaker screen opening dimensionwill pass through the screen. Wood pieces having a width dimension Wwhich is less than a screen opening size, but a length dimension L whichis greater than the screen opening size a will typically not passthrough the screen using this technique. Using this method, virtuallyall of the dust and fines that are less than ¼ inch in length, width andthickness are removed from the precursor wood pieces 518, leaving onlythe wood chips 102, which are then used for the first wood component ofthe composite material.

[0042] After grinding and screening, the wood chips will typically havea moisture content in excess of about 12% by weight. It is preferredthat the wood chips 102 of the first wood component have a surfacemoisture content of about 1% by weight or less. The desired moisturelevel is typically achieved by heating the wood chips 102 in a dryer ata temperature above 212° F. Preferably the wood pieces are heated to atemperature from about 220° F. to about 240° F. It is preferable thatthe smaller wood fines and wood particles be removed from the wood chips102 prior to drying in order to control the flammability and/orauto-ignition characteristics of the material being dried.

[0043] In the thermoplastic composite wood material 100, the wood chips102 of the first wood component and the wood particles 104 of the secondwood component constitute, respectively, the first and seconddiscontinuous phases that are distributed throughout a continuous phase,or matrix, of a thermoplastic polymer material 106. Various types ofthermoplastic polymers may be used for the continuous thermoplasticcomponent, including polypropylene, polyethylene, polyvinyl chloride(PVC), styrene and ABS, however, it has been determined thatpolypropylene is especially suitable for use in this invention. It ispreferred that a polypropylene homopolymer be used for the continuouspolymer phase 106 because such materials exhibit good melt and flowproperties. It is more preferred to use a polypropylene homopolymerhaving a melt index in the range from about 0.5 to about 12. Apolypropylene having a melt index from about 0.5 to about 2 isespecially preferred in embodiments where only relatively sturdycomponents such as wood chips 102 and wood particles 104 are used. Apolypropylene having a melt index from about 4 to about 12 is especiallypreferred in alternative embodiments where relatively fragile structuralcomponents such as fiberglass are included along with the wood chips andthe wood particles.

[0044] In one embodiment of a thermoplastic composite wood material, thewood chips 102 constitute from about 100 to about 0 weight percent ofthe total weight of the first and second wood components, and the woodparticles 104 constitute from about 0 to about 100 weight percent of thetotal weight of the first and second wood components. In thisembodiment, the first and second wood components together constitutefrom about 90 to about 50 weight percent of the composite wood material,and the thermoplastic polymer component 106 constitutes from about 10 toabout 50 weight percent of the composite wood material. It will beappreciated that the total weight percent of the first and second woodcomponents and the thermoplastic polymer component may be less than 100%of the total weight of the composite wood material to allow for theaddition of small amounts (i.e., typically less than about 5% by weightof the total) of non-structural additive materials such as paraffins,colorants, UV stabilizers, fire-retardants, impact modifiers, and thelike. In some alternative embodiments, however, a structural additive,e.g., fiberglass, may be added in considerable quantity such that thetotal weight of non-wood and non-thermoplastic components may be greaterthan about 5% by weight.

[0045] In another embodiment, the wood chips 102 constitute from about90 to about 65 weight percent of the total weight of the first andsecond wood components and the wood particles 104 constitute from about10 to about 35 weight percent of the total weight of the first andsecond wood components. In a more preferred embodiment, the wood chipsconstitute from about 85 to about 75 weight percent of the total weightof the first and second wood components, and the wood particles 104constitute from about 15 to about 25 weight percent of the total weightof the first and second wood components. In yet another embodiment, thefirst and second wood components together constitute from about 80 toabout 65 weight percent of the composite wood material and thethermoplastic polymer component 106 constitutes from about 20 to about35 weight percent of the composite wood material. In a more preferredembodiment, the first and second wood components together constitutefrom about 77 to about 70 weight percent of the composite wood material,and the thermoplastic polymer component 106 constitutes from about 23 toabout 30 weight percent of the composite wood material. A thermoplasticcomposite wood material in accordance with these embodiments will have afinal compressed density from about 30 to about 50 pounds per cubic footafter pressing.

[0046] Referring now to FIG. 6, there is illustrated a two-partthermoplastic composite wood material 600 in accordance with anotherembodiment. The composite wood material 600 comprises an innerstructural member 602 which is continuously joined to an outerstructural member 604. The inner structural member 602 is made from athermoplastic composite wood material substantially identical to thematerial 100 previously described, i.e., it includes a first woodcomponent of substantially axially aligned wood chips 102 having a firstsize range and being distributed through the inner structural member ina first continuous phase, a second wood component of wood particles 104having a second size range and being distributed between the wood chips102 in a second discontinuous phase, and a first thermoplastic polymercomponent which encapsulates the wood chips 102 of the first woodcomponent and the wood particles 104 of the second wood component,forming a first continuous phase extending throughout the firststructural member. As previously described, the second size range of thewood particles 104 is distinct from the first size range of the woodchips 102 and has substantially smaller values than the first sizerange. In the inner structural member, the first and second woodcomponents together constitute from about 90 to about 50 weight percentof the inner structural member and the first thermoplastic polymercomponent 106 constitutes from about 10 to about 50 weight percent ofthe inner structural member. As previously described for the compositewood material 100, the first structural member 602 has an axis 606running therethrough and the wood chips 102 of the first wood componentare substantially aligned therewith. Put another way, the long axes 202of the individual wood chips 102 are substantially parallel to oneanother and to the axis 606 of the inner structural member.

[0047] The outer structural member 604 is continuously joined to theinner structural member 602 along a boundary 608. The outer structuralmember 604 comprises a third wood component of wood particles 610 havinga third size range distributed throughout the outer structural member ina third discontinuous phase and a second thermoplastic polymer componentwhich encapsulates the wood particles and forms a second continuousphase 612 extending throughout the outer structural number. In thetwo-part thermoplastic composite wood material 600, the third woodcomponent constitutes from about 10 to about 50 weight percent of theouter structural member and the second thermoplastic polymer componentconstitutes from about 90 to about 50 weight percent of the outerstructural member. Wood particles 610 of the third wood component may beof various sizes, however typically they will have a size range similarto the size range of the wood particles 104 in the inner structuralmember. In some embodiments, the wood particles 610 of the third woodcomponent can be taken from the same source of supply as the woodparticles 104 of the second wood component. Accordingly, in oneembodiment of the composite wood material 600, the first size rangeincludes wood chips 102 having an axial length from about ¼″ to about 2″and a width from about {fraction (1/16)}″ to about ¾″, the second sizerange includes wood particles 104 having a diameter from about 0.010″ toabout 0.076″ (i.e., from about 60 mesh to about 9 mesh), and the thirdsize range includes wood particles 610 having a diameter from about0.010″ to about 0.076″ (i.e., from about 60 mesh to about 9 mesh). Inanother embodiment, the composite wood material 600 includes an innerstructural member having a density from about 30 to about 50 lbs. percubic foot and the outer structural member has a density from about 50to about 80 lbs. per cubic foot.

[0048] The first polymer component 106 of the thermoplastic compositewood material 600 may be any thermoplastic material as discussed for thecomposite wood material 100. The second polymer component of the outerstructural member may be the same thermoplastic material which is usedin the inner structural member. In many cases, however, the polymermaterial for the second polymer component will be selected to providethe outer structural member with different properties than the innerstructural member. For example, in one embodiment of the composite woodmaterial 600, a polypropylene homopolymer is utilized for the firstpolymer component 106 and a polypropylene copolymer is utilized for thesecond polymer component 612. It has been found that the use of thepolypropylene copolymer results in an outer structural member havinggreater impact resistance than the inner structural member.

[0049] The two part thermoplastic composite wood material 600 may beused to produce a variety of useful building materials and otherproducts. For example, referring to FIG. 7, the two part thermoplasticcomposite wood material 600 may be formed into a structural product 702which, when viewed in axial cross-section, has the outer structuralmember 604 which completely encloses the inner structural member 602.Such materials are especially well suited for exterior buildingapplications, e.g., fencing or decking, where the outer structuralmember 604 may be formulated to provide good weather and impactresistance while the inner structural member 602 is formulated toprovide maximum strength and low weight or density.

[0050] Referring now to FIG. 8, there is illustrated another productformed using the two-part thermoplastic composite wood material 600,wherein the outer structural member 604 is contiguous with one or moreedges of the inner structural member 602, but does not completelysurround the inner structural member. Further, it is illustrated thatboth the inner structural member 602 and outer structural member 604 maybe formed with various contours 804 as necessary for a particularapplication. The building product 802 and similar articles may beparticularly useful in applications such as window casings or decorativemolding, where only a portion of the material is exposed to theelements. In such cases, the weather exposed portions are covered by theouter structural member 604 to provide good weather resistance, and/orimpact resistance while the non-exposed areas 806, 808 may comprise onlythe inner structural member 602.

[0051] Referring now to FIG. 9, there is illustrated a process formanufacturing the thermoplastic composite wood materials previouslydescribed. Block 902 represents the wood raw material which will be usedto form the first, second and third (i.e., if applicable) woodcomponents. The wood material may be virgin wood or, preferably,industrial manufacturing waste wood (e.g., lumber cutoffs) or scrap wood(e.g., old wooden pallets). The raw wood material proceeds to block 904where it is first ground and screened to form properly sized wood chips.Suitable grinding and screening operations were described in detailpreviously. The resulting wood chips within the first size range proceedvia path 906 to block 908, where they are dried by heating in order toreduce the surface water content to about 1% by weight or less.Preferably, the wood chips are dried in a natural gas-fired dryer usingcombustion gases which are oxygen depleted to prevent burning of thechips. After drying, a portion of the wood chips are taken from block908 via path 914 to grinding block 912. Preferably, the wood chips aretransported from the dryer to the grinder using a side stream ofcombustion gas to further dry the chips. In block 912, the wood chipsare further ground and then screened to form particles within the sizerange for the second wood component and, if applicable, the third woodcomponent. In addition, it is preferred that the particles are furtherdried to reduce their surface moisture content to about 0.5% by weightor less.

[0052] Dried wood chips constituting the first wood component (woodPhase 1) proceed from the drying block 908 to the block 916 forcompounding. Similarly, wood particles constituting the second woodcomponent (wood Phase 2) proceed from the second grinding block 912 tothe compounding block 916. The block 918 represents the thermoplasticraw material which will be used for the first polymer component. Thefirst thermoplastic raw material may be virgin thermoplastic pellets orbeads, recycled thermoplastics, or a combination of both. In addition,plastic additives (e.g., impact modifiers, anti-oxidants, UVstabilizers, colorants, fire-retardants, etc.) may be added to theunmelted plastic feedstock if it is desired to modify the properties ofthe first thermoplastic component. The first thermoplastic raw materialand any plastic additives proceed to block 920 where they are mixed,heated until molten and then extruded in a molten state into thecompounding operation represented by block 916. In the compoundingoperation 916, the first and second wood components are thoroughly mixedtogether with the molten first thermoplastic component until the woodchips and wood particles are encapsulated or thoroughly coated by thethermoplastic material. In addition, structural additives such as glassfibers may be added during the mixing/compounding operation 916. A smallamount of paraffin wax additive (typically about 0.5 to 1.0% by weight)maybe added into the compounder to improve processing of the wood chips.It is believed that the paraffin acts in several ways: first, as alubricant allowing the wood chips and particles to move more easilyrelative to one another for orientation and alignment; second, as a heattransfer medium; and third, as an agent to enhance the flow of thepolymer around the wood pieces. The paraffin wax may be added in amolten state by spraying it into the compounder using spray nozzles.

[0053] After the wood chips 102 and wood particles 104 have beenencapsulated with the thermoplastic material, in the mixing step 916,the wood chips are then oriented such that their long axes 202 areparallel to a predetermined plane. This step of the process isrepresented by block 922. The orientation step 922 does not necessarilyresult in the wood chips 102 being axially aligned at this point, forthe long axes 202 in the various chips maybe directed at various anglesto one another as long as they are parallel to the predetermined plane.

[0054] Referring now to FIGS. 10a and 10 b, two chips (denoted 102′ and102″) of the first wood component are shown in relation to a plane 1002.As best seen in FIG. 10a, the axes 202 of the chips are not parallel toone another, however, as best seen in FIG. 10b, the axes are bothparallel to the predetermined plane 1002. This is an example of the woodchip orientation performed in step 922 of the process of FIG. 9.

[0055] While the mixing step 916 and the chip orientation step 922 maybe carried out in separate operations, in a preferred embodiment of theprocess these steps are performed in a single continuous operation. In amore preferred embodiment, the mixing step 916 and the chip orientationstep 922 are performed in a single compounder unit. It will beappreciated that the orientation of the wood particles 104 constitutingthe second wood component may or may not be affected by the orientationprocess 922, however this is not an issue since no particularorientation of the wood particles is required in the thermoplasticcomposite wood material.

[0056] The mixture of oriented wood chips 102, wood particles 104, andencapsulating molten thermoplastic next passes out of the mixing and/ororienting unit and is deposited onto the inlet feed of a directionalpress unit. This step is represented by block 924. It is significantthat the orientation of the wood chips 102 achieved in step 922 ismaintained as the mixture is deposited. In a preferred embodiment, thetemperature and composition parameters of the thermoplastic-wood mixtureare controlled to produce a loose material, i.e., a material havingdiscreet nodules or “clumps” of material rather than a continuousstrand.

[0057] The loose material comprising oriented wood chips 102, woodparticles 104 and still-molten thermoplastic next moves to a directionalpress unit, denoted by block 926. The directional press unit appliesforce to the loose material, simultaneously compressing it and causingthe oriented wood chips 102 to become substantially axially aligned. Inthis case, substantially axially aligned means that the axes 202 of theindividual wood chips are substantially parallel to one another.Referring now to FIGS. 11a and 11 b, the two wood chips 102′ and 102″are shown moving from their oriented configuration (shown in brokenline) with axes 202 parallel to plane 1102, to an axially alignedconfiguration with axes 202 parallel to one another, under the influenceof the directional forces 1104 applied by the directional press. It willof course be appreciated that the mixture must be constrained frommoving laterally during the pressing operation to efficiently achievethe axially aligned condition for the first wood component. After thedirectional pressing operation 926, the basic thermoplastic compositewood material is essentially complete as indicated in block 928. If asingle-part composite product is required, then the finished materialfrom block 928 may then be cooled until the thermoplastic componenthardens as indicated by block 930. The finished material may also besubjected to various finishing operations, for example embossing thematerial surface with a decorative pattern, surface finishing, coding ormarking.

[0058] Alternatively, if a two-part composite wood material is desired,i.e., one having an inner structural member and an outer structuralmember as previously described and illustrated in FIG. 6, then the basicprocess just described must be expanded. Referring still to FIG. 9, thethermoplastic raw material of the second polymer component isrepresented by block 932. Plastic additives (e.g., impact modifiers suchas linear low-density polyethylene, anti-oxidants, UV stabilizers,colorants, fire-retardants, etc.) may be added to the unmelted plasticfeedstock if it is desired to modify the color and/or properties of theouter structural member. This material is passed to an extruder 934where it is mixed, heated until molten, and then extruded into a mixingoperation 936. Meanwhile, the wood particles 610 of the third woodcomponent are prepared in the grinding and screening operation 912 andthen moved to the mixing operation 936 via path 938. The mixingoperation 936, which is preferably accomplished using a compounder unit,thoroughly mixes the wood particles 610 and the molten thermoplasticuntil the particles are encapsulated. As with the first compounder step916, a small amount of paraffin wax additive (typically about 0.5% byweight) may be added into the second compounder step 936 to improveprocessing of the wood particles. The resulting mixture of thermoplasticand encapsulated wood particles 610 then proceeds to the applicationoperation, represented by block 940. In the application operation 940,the molten mixture of the second thermoplastic component 612 and thethird wood component 610 is continuously joined to the previouslycompleted one-part composite wood material from block 928. Theapplication of the outer structural member may completely enclose theinner structural member, or it may only partially enclose the innerstructural member. The application process may be accomplished byconventional extrusion of the outer material around the inner materialusing a hot melt extruder and profile molder, by the use of calenderforming rolls to form a sheet of outer material for application to theinner structural member or by the use of newly developed coating dieequipment as further described herein.

[0059] Referring now to FIG. 12, there is illustrated the layout for aplant suitable for manufacturing a thermoplastic composite wood materialin accordance with another embodiment of the current invention. Woodchips of suitable size for the first wood component are stockpiled in awood bin 1202. The wood chips 102 are transferred as needed to a drierunit 1204 via transfer augers 1206 and 1208. The drier 1204 preferablyuses combustion gases (i.e., gases resulting from combustion that havelittle or no remaining free oxygen) at an elevated temperature to drythe chips until they have a moisture content of about 1 weight percentor less. In a preferred embodiment the drier is fired with natural gas.The dried chips 102 are then transferred via auger 1210 to a bin 1212.Some of the wood chips are taken from the bin 1212 via auger 1222 to agrinding mill 1216 to produce the wood particles 104 for the second woodcomponent. In this embodiment, the wood particles 610 for the third woodcomponent have the same size range as the wood particles 104, and thusare produced in the same mill 1216. It will be appreciated that anothergrinding mill could be provided if the wood particles 610 had adifferent size range. Preferably, a side stream of hot (i.e., typicallyabout 250° F.) combustion gases are taken from the dryer 1204 to thegrinding mill 1216 via a duct 1214 to further dry the wood particlessuch that their moisture content is about 0.5 weight percent or less.These gases are further used to transport the dried wood particles fromthe grinding mill 1216 to a cyclone unit 1218 via a duct and blower1217. The cyclone unit 1218 separates the wood particles from thecombustion gases, routing the combustion gases back to the dryer 1204via a duct 1220, and routing the dried wood particles to meteringdevices 1221 and 1223. The metering devices 1221 and 1223 control theflow of the wood particles from the cyclone unit 1218 to the respectivecompounders so as to maintain the proper proportion of components. Thefirst and second wood components are then transferred, respectively,from the bin 1212 and the metering unit 1221 via auger 1224 to an innercompounder unit 1226. As previously described, a small amount ofparaffin wax additive (typically about 0.5 to about 1% by weight) maybeadded directly into the inner compounder at this point to improveprocessing of the wood components and to act as a heat transfer mediumin the compounder. It will be appreciated that the amount of woodparticles 104 present in the wood components introduced into the innercompounder 1226 may be controlled by a metering device, e.g., meteringdevice 1221, to determine the final compressed density of the innerstructural member. Glass fibers or other structural additives may alsobe added into the compounder to achieve certain physical properties ofthe inner structural member.

[0060] The thermoplastic raw material for the first polymer component isintroduced into an inner extruder 1228. In addition, plastic additives,for example, impact modifiers, anti-oxidants, UV stabilizers, colorants,and fire-retardants, may be added to the unmelted plastic feedstock inthe extruder if desired. The inner extruder 1228 then mixes thefeedstock (and additives, if present) and heats it above its meltingpoint. The molten thermoplastic material is injected into the innercompounder 1226 where it is compounded with the wood chips 102 and woodparticles 104 at a temperature less than the ignition temperature of thewood pieces (typically less than about 400° F.). While not used in thisembodiment, in alternative embodiments glass fibers or other structuraladditives may also be added into the compounder for mixing with thethermoplastic and wood particles. The wood chips 102, wood particles 104and thermoplastic materials are compounded, or mixed, together in thecompounder 1226 with a minimal size reduction of the wood pieces. Inother words, the compounder 1226 is designed to minimize any grinding,breaking or shredding of the wood chips 102 such that the originaldesired dimensions are maintained. The compounder 1226 performs twoessential functions in the process. First, it thoroughly mixes the woodchips 102, wood particles 104 and melted thermoplastic material so as tothoroughly encapsulate the wood pieces with thermoplastic. Secondly, theinner compounder 1226 serves to orient the random alignment of the woodchips 102 such that their long axes are perpendicular to the directionof material flow.

[0061] Referring now to FIGS. 13 and 14, there is illustrated adiagrammatic view of a compounder unit suitable for performing thecompounding and orienting operations previously described. FIG. 13 showsa top view of the inner compounder 1226 having two feed screws 1302 and1304 positioned in a parallel configuration. The compounder 1226 has awood feed section 1306 and a thermoplastic feed section 1308 foraccepting their respective components and moving them through thecompounder in a flow direction denoted by arrow 1310. It will beappreciated that the relative positions of the wood feed section 1306and the thermoplastic feed section 1308 may be reversed if desired,i.e., the relative order of introduction of the wood components and thethermoplastic components into the compounder is typically not a criticalfactor provided the components are thoroughly mixed. The componentmaterials are transported by the feed sections 1306, 1308 into acompounding section 1312 having mixing elements for the compounding ofthe components until the wood chips and wood particles are thoroughlyencapsulated by the molten thermoplastic. The various compoundercomponents are generally selected to minimize, and preferably eliminate,any crushing, grinding or other size reduction of the wood componentsbeing compounded.

[0062] It will be understood that the wood feed section 1306,thermoplastic feed section 1308 and compounding section 1312 of thecompounder unit 1226 are generally conventional in nature. Thecompounder unit 1226 of the current embodiment, however, furtherincludes an orientation section 1314 for orienting the previously randomalignment of the wood chips 102 in a direction perpendicular to thematerial flow 1310. This orientation is accomplished by equipping thecompounder shafts 1302 and 1304 with screw elements 1316 having a screwpitch which becomes progressively smaller as the distance to thecompounder outlet is reduced. In other words, as the wood pieces 102move through the orientation section 1314 in the direction of materialflow 1310, the screw elements 1316 will become closer together whilesimultaneously assuming an orientation which is closer to perpendicularwith respect to the material flow. This will cause the wood chips 102 ofthe first wood component to become oriented with their long axes 202substantially parallel to a predetermined plane, e.g. the plane 1318which is perpendicular to the material flow direction 1310. An outletopening 1320 is provided on the lower end of the compounder unit 1226,allowing the mixed and oriented components to exit the compounder whilemaintaining the orientation of the wood chips 102.

[0063] Referring now to FIG. 14, there is illustrated a side view of theinner compounder 1226. The compounder exit opening 1320 is equipped withan adjustable slide gate 1402 which serves to control the flow rate ofmixture leaving the compounder 1226 and also further orients the woodchips 102 so that their axes 202 are generally parallel to apredetermined plane, or put another way such that the axes 202 aregenerally perpendicular to the direction of material flow 1310.

[0064] At a given feed rate of materials into the inner compounder 1226,and at a given RPM of the feed screws 1302 and 1304, the adjustableslide gate 1402 is set to control the compression and compounding/mixingintensity that occurs within the inner compounder. The inner compoundermay have either heating or cooling jackets in order to maintain thedesired conditions within the compounding section 1312. The compressedand compounded materials are discharged from the inner compounderthrough the exit opening 1320 as indicated by arrow 1404 and depositedon an inlet conveyor 1230 (FIG. 12) where they are allowed to expand toan uncompressed state.

[0065] While the compounded and oriented mixture produced by the innercompounder 1226 may exit as a continuous flow or strand of material, itpreferably has a discontinuous consistency similar to that of loosefiberglass insulation. This material is transported by the press inletconveyor 1230 to a continuous press 1232 where the material will becompressed to its final density and the wood chips 102 aligned to givethe resulting composite wood material its favorable strength and bendingcharacteristics.

[0066] Referring now to FIG. 15, there is illustrated a diagrammaticside view of a continuous press 1232 suitable for use in this process.In the particular embodiment illustrated in FIG. 12, the continuouspress 1232 operates at a 90° angle from the direction of material flowthrough the inner compounder 1226. The compounded material exiting theinner compounder 1226 falls vertically downward from the exit opening1320 across a distributor plate 1502, which moves back and forthlaterally to distribute the falling material (denoted 1504) across thewidth of the conveyor inlet section 1506. It will be appreciated thatthe orientation of the wood chips 102 in the falling mixture 1504 isgenerally maintained as they are distributed on the conveyor to form anuncompressed mass (denoted 1508). The feed conveyor 1230 moves theuncompressed mass 1508 across a horizontal transfer plate 1510 and intothe space between upper and lower compression rollers 1512 and 1514. Theupper and lower rollers 1512 and 1514 turn in opposite directions (asdenoted by arrows 1516 and 1518) and are synchronized to rotate at thesame speed as the travel of the compounded material 1508 on theconveyor. As the material passes between the rollers 1512 and 1514, itis compressed and the oriented wood chips 102 become aligned in thehorizontal plane, i.e., parallel to the direction of travel (denoted byarrow 1520) of the compounded material. After exiting the continuouspress, the compressed aligned material (denoted 1522) may enter aretainer tube 1524 which maintains the profile of the material until thethermoplastic component cools and hardens (if a one-part compositematerial is being produced), or until the outer structural member isapplied (if a two-part composite material is being produced). In otherwords, the compressed aligned material 1522 is nearly finishedthermoplastic composite wood material of the single part form, requiringonly cooling, cut off, and surface finishing (if desired) to becomplete.

[0067] Referring now to FIGS. 16a and 16 b there is illustrated furtherdetails of a continuous press 1232. As best seen in FIG. 16a, the space1602 between the opposing faces of the rollers 1512 and 1514 is termedthe inner profile, as it controls the profile of the resulting innerstructural member. The continuous press has side members 1604, 1606,1608 and 1610 that define the width of the inner profile 1602. Inaddition, side plates (not shown) are provided on each side of thematerial flow path to confine and guide the loose material as it travelson the conveyor 1230, passes through the continuous press 1232, and goesinto the retainer tube 1522. The rollers 1512 and 1514 have slots 1611on each side of the roller, i.e., between the roller and the sidemembers 1604, 1606, 1608 and 1610, to accommodate the side plates asthey pass through the continuous press. In a preferred embodiment, theside members 1604, 1606, 1608 and 1610 are adjustable to allow the widthof the inner profile 1602 to be changed. Preferably, the verticalspacing between the rollers 1512 and 1514 can be adjusted to change theheight of the profile and/or to change the amount of compression exertedon the uncompressed material 1508. As best seen in FIG. 16b, a pair ofintermeshing gears 1612 and 1614 maybe provided on the roller shafts1616 and 1618 in order to synchronize movement of the rollers 1512 and1514 with one another and with the motion of the composite wood mixtureas it passes through the inner profile 1602. In other embodiments,sprockets connected with a chain or toothed belt may be used tosynchronize the rollers of the continuous press.

[0068] As previously described, the thermoplastic composite woodmaterial 1522 which leaves the continuous press 1232 may represent thedesired final product, e.g., a building material for fencing or decking.If plastic additives such as colorants, anti-oxidants, UV stabilizers,etc. were added to the plastic feedstock in the inner extruder, then thecomposite wood material may need little finishing other than beingcooled, having a surface decoration added (if desired) and being cut tolength. However, since the wood chips 102 may be exposed or very nearthe surface of the material, the surface finish of such a product may berough or unsuitable for exterior usage. For similar reasons, thesingle-part composite wood material may not have the desired impact orweather resistance for certain applications. When it is desired toproduce a two-part thermoplastic composite wood material, i.e., havingan outer structural member joined to the inner structural member whichexits the continuous press 1232, then additional plant equipment isrequired as described below.

[0069] Referring again to FIG. 12, the thermoplastic raw material forthe second polymer component is introduced into an outer extruder 1234.As previously described, this second polymer component may be the sametype of thermoplastic used for the first polymer component, or it can bea different type of thermoplastic. Further, as previously described forthe inner extruder, plastic additives such as colorants, impactmodifiers (e.g., linear low-density polyethylene), anti-oxidants, UVstabilizers, and fire retardants may be added to the unmelted secondplastic feedstock in the outer extruder 1234. The thermoplasticfeedstock and any additives are then mixed and heated above its meltingpoint in the outer extruder 1234. The molten thermoplastic is introducedby the outer extruder 1234 into an outer compounder 1236. Wood particles610 making up the third wood component are also introduced into theouter compounder 1236 from the metering device 1223. While not used inthis embodiment, in alternative embodiments glass fibers or otherstructural additives may also be added into the compounder for mixingwith the thermoplastic and wood particles. The melted thermoplastic andwood particles 610 are compounded in the outer compounder 1236 until thewood particles are encapsulated by the thermoplastic. The outercompounder will typically be insulated or heated to maintain thethermoplastic in a molten state and reduce its viscosity as much aspossible, however, the temperature of the materials in the outercompounder 1236 must remain below the ignition temperature of the woodpieces.

[0070] The mixture produced in the outer compounder 1236 will become theouter structural member of the two-part composite wood material,however, it must first be applied to the inner structural member(denoted 1240) produced by the continuous press 1232. It will beappreciated that in this embodiment, the inner structural member 1240 isthe compressed aligned material 1522 of FIG. 15. To produce the two-partcomposite material, the inner structural member 1240 and the mixturefrom the outer compounder 1236 are first delivered to an applicator unit1242. As will be described in further detail below, several types ofapparatus may be used for the applicator unit 1242, including aconventional extruder die unit (FIG. 17), a calender forming roll unit(FIGS. 18a and 18 b), or a newly developed coating die unit (FIGS. 19a,19 b, 20 a and 20 b).

[0071] After the outer structural member is applied (or, for theone-part composite wood, after the material exits the continuous press),the material proceeds for final processing. Typically, final processingincludes cooling the material, applying surface decoration if desired,and cutting to length. The cooling operation may include running thecomposite wood material through a water bath 1246 and/or chilled waterspray until the thermoplastic material has solidified throughout amajority of the material. Surface decorations, e.g., wood grainpatterns, etc., may be applied to the surface of the material using anembossing roll, heated if necessary. The cut-off operation cuts thematerial to length and maybe accomplished using conventional cut-offtechnologies. However, it is preferred that a water-proof orwater-resistant coating be applied to the “raw” cut end of the materialimmediately after the cut-off operation to prevent the exposed woodchips from absorbing water from subsequent cooling processes or fromsubsequent environmental exposure. In one preferred embodiment, alatex-based material is sprayed onto the cut end of the materialimmediately after cut-off to provide water resistance. It will beappreciated that these and other finishing procedures may be practicedin various orders without departing from the scope of the currentinvention.

[0072] Referring now to FIG. 17, there is illustrated an outer profilemolder 1702 of a type suitable for applying the outer structural memberto the inner structural member 1240. The outer profile molder 1702typically includes an inner member retainer tube 1703 for positioningthe inner structural member 1240 as it enters the unit. A temperaturecontrolled reservoir cavity 1704 is provided holding a given quantity ofthe thermoplastic-wood mixture produced in the outer compounder 1236.The outer compounder 1236 must supply the outer mixture to the supplycavity 1704 at a rate sufficient to keep the cavity full. Adequatepressure must be maintained in the outer profile molder 1702 in order toapply the outer structural member 1244 over the inner structural member1240. If the compounder 1236 cannot provide sufficient pressure, then asupplemental extruder (not shown) may be required to inject the moltenmixture into the outer profile molder. As the inner structural member1240 passes through the outer profile molder 1702, the surfacetemperature of the inner structural member is raised, allowing a thermalbond to occur between the thermoplastics of the inner and outerstructural members. Adjustable slide gates 1706 may be used to controlthe flow rate of the outer material as it is applied to each side of theinner structural member 1240. The outer profile molder 1702 utilizesconventional extruder technology to form the two-part thermoplasticcomposite wood material. The use of this technology is well known, andit works relatively well for the thermoplastic composite wood of thisinvention where the thickness of the outer structural member 604 iswithin the range from about 0.010 inch to about 0.040 inch. For thickercoatings, the operating costs tend to be comparatively high, due largelyto the fact that typical thermoplastics (e.g., polypropylene) have arelatively high viscosity at temperatures below 400° F. As previouslydiscussed, 400° F. is the approximate temperature at which the woodcomponents in the outer mixture will ignite or break down. Injecting orextruding the highly viscous mixture of (relatively) low temperaturemolten thermoplastic and wood particles into the outer profile molder1702 tends to be very energy intensive.

[0073] Referring now to FIGS. 18a and 18 b, there is illustrated analternative applicator unit which utilizes coating rollers rather thanan outer profile molder to apply the outer structural member 1244 to theinner structural member 1240. In this embodiment, the outer compounder1236 is equipped with a die plate 1802 which splits the material exitingthe compounder into two continuous strands 1804 and 1806. The strandsare at a temperature at which they are still melted and flexible, yetable to withstand limited tensile stress. The strands 1804 and 1806 arerouted by a series of strand guide rollers 1808 to pairs of closelyspaced calender rolls 1810 and 1812. The mixture strands 1804 and 1806are flattened as they pass between the calender rolls 1810 and 1812,forming a wide, thin film 1814 of outer material. This film 1814 is thenrouted by a series of film guide rollers 1816 and then pressed onto theupper and lower surfaces of the inner structural member 1240 byapplication rollers 1818 and 1820. If the film 1814 is initially widerthan the inner structural member 1240, then the “overhanging” edges 1822of the film may be folded down onto the sides of the inner structuralmember by side rollers (not shown) to completely enclose the innerstructural member with the outer structural member. It will beappreciated that the inner and/or outer structural members must bemaintained at a temperature sufficient to achieve a thermal bond betweenthe thermoplastics of the inner structural member and the outerstructural member. The use of calender forming rolls allows the outerstructural member 1244 to be applied at higher throughput rates and withlower energy usage than the conventional extrusion technologyrepresented by the outer profile molder previously discussed.

[0074] Referring now to FIGS. 19a and 19 b there is illustrated yetanother type of alternative applicator unit suitable for applying theouter structural member to the inner structural member. The apparatus,called a coating die unit, is also suitable for other applicationsinvolving the extrusion of high viscosity materials. The coating dieunit 1902 comprises an upper coating die 1904, a lower coating die 1906,calender rollers 1908 and application rollers 1910. Molten material fromthe outer compounder 1236 (FIG. 12) is supplied to each coating die1904, 1906 via supply pipes 1914. In some cases, a supplemental boostpump 1912 is provided as shown to increase the flow rate of moltenmaterial from the outer compounder 1236 to the coating dies 1904, 1906.A raw sheet 1916 of molten material is dispensed from each coating die.It is significant that each raw sheet 1916 dispensed from the coatingdie 1904, 1906 has a thickness dimension and a width dimension that arevery close to the desired finished dimensions of the sheet. The rawsheet 1916 then passes between calender rolls 1908 which reduce thethickness of the sheet to form a final sheet 1918. The calender rollsare typically heated to bring the temperature of the final sheet 1918 toa temperature sufficient to cause thermal bonding with the innerstructural member, e.g., typically about 315° F. to about 320° F. Insome embodiments, a supplemental heat source, for example, IR lamp 1921,is provided to specifically heat the surface of the sheet 1918 and thesurface of the structural member 1240 just before contact to ensure agood bond. The hot final sheets 1918 are then guided by the applicationrollers 1910 which press the final sheets against the top and bottom ofthe inner structural member 1240 as it passes between the rollers in thedirection indicated by arrow 1920. The hot sheets 1918 then bond to thetop and bottom of the inner structural member 1240. As best seen in FIG.19b, the final sheet 1918 has a width dimension wider than the width ofthe inner structural member 1240, such that a portion of the sheet“overhangs” on each side of the inner structural member. A pair of siderollers 1922 or guides folds the overhanging material onto the sides ofthe inner structural member 1240. The overhanging material from the topsheet is dimensioned to overlap the overhanging material from the bottomsheet when folded down. Thus, as the side rollers press the overlappingmaterial against the side of the inner structural member, the hotmaterial bonds to the inner structural member and to the other sheet,forming a continuous coating of the outer structural member around theinner structural member.

[0075] Referring now to FIGS. 20a and 20 b, there is illustrated anenlarged view of the upper coating die 1904, it being understood thatthe lower coating die 1906 is substantially identical. The coating die1904 includes an exterior casing 2002 which defines a longitudinalcavity 2004. A single shaft 2006 is mounted in each cavity 2004 androtated by an external power source (not shown). The shaft 2006 isequipped with paddles or screws 2007 thereon which define a feed section2008 and a dispensing section 2010. The paddles 2007 in the feed section2008 have a positive pitch that forces the molten material received fromthe compounder 1236 into the dispensing section 2010 with sufficientforce and volume to keep the dispensing section full. A tangentiallydisposed dispensing slot 2012 is formed through the exterior casing 2002and extends longitudinally across the dispensing section. The paddles2007 in the dispensing portion 2010 push the molten material 2009 out ofthe cavity through the dispensing slot to form the raw sheet 1916 (FIGS.19a and 19 b). Preferably, the paddles 2007 along the shaft 2006 areangularly offset from one another in the dispensing section 2010, asthis serves to keep the mixture 2009 moving properly through the coatingdie. It is significant that the dispensing slot 2012 is formedtangentially through the exterior casing 2002 as shown. As best seen inFIG. 20a, the exposed interior edge 2014 of the slot 2012 serves as a“knife” which continuously slices a sheet of the molten material 2009from the rotating mass within the coating die. The mechanical cutting ofthe sheet material in the coating die 1904 is much more energy efficientthan forcing the material through a slot with pressure alone, as inconventional extruding technologies.

[0076] In one example of this invention, an extruded product will bemade from the two-part thermoplastic composite wood material. Thefinished product will have overall dimensions of 5.50″×1.50″ (i.e.,nominal 2×6 lumber size) including a 0.125″ thick outer structuralmember that completely surrounds the inner structural member. The rawmaterial feed rates for the inner structural member are as follows: 1064lbs/hr of dried wood chips having an axial length from about ¾″ to about1-¼″ and a width from about ⅛″ to about ¼″; 267 lbs/hr of dried woodparticles having a diameter from about 0.023″ to about 0.033″; and 492lbs/hr of polypropylene pellets. The dried wood pieces are fed into theinner compounder at a temperature of about 220° F. and the moltenpolypropylene is fed into the compounder from the extruder at atemperature of about 395° F. This results in an inner compounder exittemperature of about 267° F. The material is dispensed as loose clumpsform the inner compounder and compressed in the continuous press to forman inner structural member with a density of about 39.6 lbs/cu. ft. Thecorresponding raw material feed rates for the outer structural memberare as follows: 329 lbs/hr of dried wood particles having a diameterfrom about 0.023″ to about 0.033″; and 402 lbs/hr of polypropylenepellets. The dried wood particles are fed into the outer compounder at atemperature of about 220° F. and the molten polypropylene is fed intothe compounder from the outer extruder at a temperature of about 385° F.This results in an outer compounder exit temperature of about 311° F. Acoating die is used to dispense the outer material received from theouter compounder. The temperature of the outer material leaving thecoating die is still about 311° F. The outer material is heated usingthe calender rollers of the coating die unit to bring the surfacetemperature of the sheet to about 320° F. The outer material (at about320° F.) is then applied, i.e., pressed against the inner structuralmember (still at about 267° F.), causing the inner and outer structuralmembers thermally bond since they both remain in the molten state. Thematerial is then cooled to a final temperature of about 100° F. using awater bath. This process produces approximately 2554 lbs/hr of finishedmaterial having an overall density of about 43.8 lbs/cu. ft. It has beendetermined that melting the thermoplastic components, mixing them withthe wood components, and forming the resulting mixture using theprocesses and apparatus of this invention requires only about 25% of theelectrical energy required for melting, mixing and forming the sameamounts of material using conventional extrusion technologies. It willbe appreciated that this results in a significant savings ofelectricity, which may be accompanied by a savings in the cost ofmanufacture.

[0077] Although the preferred embodiment has been described in detail,it should be understood that various changes, substitutions andalterations can be made therein without departing from the spirit andscope of the invention as defined by the appended claims.

What is claimed is:
 1. A method of manufacturing a thermoplastic composite wood material comprising the steps of: mixing together a first quantity of a first wood component of wood chips having long axes and a first size range, a second quantity of a second wood component of wood particles having a second size, the second size range being distinct from the first size range and having substantially smaller values than the first size range, and a third quantity of a first thermoplastic polymer component of molten thermoplastic polymer until substantially all of the wood chips and the wood particles are encapsulated by the thermoplastic polymer; orienting the long axes of the wood chips of the first wood component such that they are substantially parallel to a predetermined plane; depositing a loose material constituting a mixture of the wood chips encapsulated in the thermoplastic polymer and the wood particles encapsulated in the thermoplastic polymer onto a press inlet feed unit while maintaining the orientation of the long axes of the wood chips; and pressing the loose material in a direction substantially parallel to the predetermined plane such that it is compacted and such that the long axes of the wood chips are aligned substantially parallel to one another.
 2. A method in accordance with claim 1, further comprising the steps of: forming the material resulting from the pressing step into an inner structural member; mixing together a fourth quantity of a third wood component of wood particles having a third size range and a fifth quantity of a second thermoplastic component of molten thermoplastic until substantially all of the wood particles of the third wood component are encapsulated by the thermoplastic polymer of the second thermoplastic component; and continuously joining the material created by mixing the third wood component and the second thermoplastic component to the inner structural member to form an outer structural member.
 3. A method in accordance with claim 2, wherein the outer structural member completely surrounds the inner structural member when the thermoplastic composite wood material is viewed in cross section.
 4. A method in accordance with claim 2, wherein the outer structural member does not completely surround the inner structural member when the thermoplastic composite wood material is viewed in cross section.
 5. A method in accordance with claim 2, wherein the step of continuously joining the outer structural member to the inner structural member is performed using a hot melt extruder and profile molder.
 6. A method in accordance with claim 2, wherein the step of continuously joining the outer structural member to the inner structural member further comprises: using calender forming rolls to form the material created by mixing the third wood component and the second thermoplastic component into a thin sheet; and pressing the thin sheet onto the inner structural member using rollers.
 7. A compounder unit for mixing and orienting shaped pieces within a viscous material, the compounder unit comprising: an outer casing having exterior walls defining a longitudinal cavity therein, the cavity being subdivided into a material inlet section, a mixing section, an orientation section, and an outlet passage and having a long axis passing therethrough; at least one compounding shaft being positioned within the longitudinal cavity parallel to the long axis, the compounding shaft having a plurality of blades formed thereon; wherein the blades on a portion of compounding shaft within the orientation section include screw blades having a pitch which progressively decreases as the distance from the blade position to the outlet passage decreases.
 8. A compounder unit in accordance with claim 7, wherein: two compounding shafts are positioned within the longitudinal cavity and parallel to the long axis; and the blades of the two compounding shafts work cooperatively to transfer the shaped wood pieces and molten thermoplastic through the inlet section, to mix the wood and thermoplastic components within the mixing section, and to orient the shaped wood pieces such that their axes are parallel to a predetermined plane.
 9. A coating die apparatus for extruding a viscous material, the apparatus comprising: an exterior casing that defines a longitudinal cavity; a shaft rotatably mounted in the cavity, the shaft having a plurality of mixing elements mounted thereon defining a feed section and a dispensing section; the mixing elements in the feed section having a positive pitch for urging molten material received into the feed section of the cavity into the dispensing section of the cavity; the mixing elements in the dispensing section urging molten material received in the dispensing section tangentially against the interior of the exterior casing; the exterior casing further defining a dispensing slot formed tangentially through the exterior casing and extending longitudinally across the dispensing section; wherein molten material received into the dispensing section is sliced off by the exposed inner edge of the casing to form a raw sheet of molten material which exits the dispensing slot.
 10. A coating die apparatus in accordance with claim 9, wherein the exterior casing has a cylindrical cross section when viewed from an end.
 11. A coating die apparatus in accordance with claim 10, wherein the mixing elements in the dispensing section comprise a plurality of rectangular paddles mounted on the shaft such that longitudinally adjacent paddles are angularly spaced apart from one another. 